1) Field of the Invention
The present invention relates to a process for producing a dialkyl carbonate, and specifically, to a process for effective utilization of allophanate by-produced during reaction or at a purifying stage in the production of a dialkyl carbonate comprising reacting urea and alcohol. The dialkyl carbonate produced according the process of the present invention is useful as a raw material of diaryl carbonate.
2) Prior Art
Japanese Patent Kokai (Laid-open) No. 55-102542 describes a process for producing a dialkyl carbonate by reaction of urea and alcohol. Japanese Patent Kokai (Laid-open) Nos. 55-102543, 57-175147 and 57-26645 describe processes for producing a dialkyl carbonate by reaction of alkyl carbamate and alcohol and further Japanese Patent Kokai (Laid-open) Nos. 10-109960, 10-259163, 10-259166 and 11-60541 disclose improved processes thereof. However, in above-mentioned prior art gazettes, the production of a by-product and its component have been not known.
The inventors have found that a solid substance having an indistinct structure is produced in the production of a dialkyl carbonate from urea and alcohol and deposited on a condenser of a distillation column and pipes in its vicinity. When operation of the distillation column was performed without removing it, there caused problems that pipes were blockaded to prevent a flow of liquid and accurate flow rate was not indicated due to its deposition on a flow meter. Thus, it was necessary to remove the solid substance with a strainer or in a settling vessel. Further, the inventors analyzed the solid substance and found that it is allophanate. However, properties of allophanate were not known in detail. Therefore, any method except waste disposal of allophanate was not found. Since allophanate is by-produced from urea or alkyl carbamate of a raw material, waste disposal of allophanate to be by-produced caused lowering unit consumption of raw material.
From the above-mentioned viewpoints, an object of the present invention is to provide a process for effective utilization of allophanate by-produced and to eliminate an apparatus for removing allophanate by-produced and operation for removal thereof.
As a result of extensive studies to utilize effectively allophanate as a by-product which has been waste disposed hitherto, the inventors have found that allophanate can be used as a raw material instead of urea or together with urea in the production of a dialkyl carbonate and furthermore can be returned to a reactor for production of a dialkyl carbonate in the state of an alcohol solution or a slurry without performing separation, and have accomplished the present invention.
That is, the present invention provides a process for producing a dialkyl carbonate which comprises performing reaction of allophanate represented by the following general formula (1) and an alkyl alcohol represented by the following general formula (2) as raw materials, thereby producing a dialkyl carbonate represented by the following general formula (3).
ROxe2x80x94COxe2x80x94NHxe2x80x94COxe2x80x94NH2 xe2x80x83xe2x80x83(1)
ROHxe2x80x83xe2x80x83(2)
ROxe2x80x94COxe2x80x94ORxe2x80x83xe2x80x83(3)
wherein R is an alkyl group.
Further, the present invention provides a process for producing a dialkyl carbonate which comprises performing reaction of urea and/or an alkyl carbamate represented by the following general formula (4) and an alkyl alcohol represented the following general formula (2) as raw materials, thereby producing a dialkyl carbonate represented by the following general formula (3), wherein allophanate represented by the following general formula (1) to be produced as by-product is reused as one of raw materials.
ROxe2x80x94COxe2x80x94NHxe2x80x94COxe2x80x94NH2xe2x80x83xe2x80x83(1)
ROHxe2x80x83xe2x80x83(2)
ROxe2x80x94COxe2x80x94ORxe2x80x83xe2x80x83(3)
ROxe2x80x94COxe2x80x94NH2xe2x80x83xe2x80x83(4)
wherein R is an alkyl group.